1. Technical Field
The invention relates to nuclear fuel handling machines and the methods for using such machines. It is particularly suitable for reloading nuclear fuel assemblies in a reactor after they have undergone a first operating cycle which has left a sufficient fissile material content for them to be used again in another location of the core.
2. Prior Art
By way of example, in pressurized water reactors the core is formed by adjacent fuel assemblies of square cross-section disposed vertically and side by side. An advantageous method of operating such a reactor consists in dividing the core into three zones with different enrichment (each having for example 52 fuel assemblies). At the end of an operating cycle of the reactor, the spent fuel assemblies in one of the zones are withdrawn for reprocessing and are replaced with assemblies taken from another zone. The assemblies withdrawn from this second zone are replaced with other assemblies having undergone one operating cycle in the reactor. And, finally, the third zone is loaded with new assemblies. The transfers of irradiated fuel assemblies take place when the reactor is shut down and cold, after the cover of the vessel has been raised and the level of water, containing boric acid, in the pool surrounding the reactor has been brought to a sufficient level for biological protection and removal of the residual heat released by the assemblies.
This method of refuelling optimizes the fuel cycle. Reloading may be carried out with visual monotoring or using immersed TV cameras. Generally, reloading is carried out with a handling machine of the type having a horizontally movable platform and carrying a vertical telescopic mast whose lower part has a gripper for gripping the upper end piece or nozzle of an assembly.
When the fuel assemblies are of great length, which is quite frequent at the present time, inserting an irradiated fuel assembly may be impeded by the deformations which the assembly may have undergone during its first cycle in a reactor. Differential expansion of the sheaths in a typical fuel assembly four meters long having a square cross-section of 214 mm per side under irradiation may lead to axial buckling resulting in a laterial offset of 20 min. The insertion of the lower end piece of the assembly in the centering studs conventionally provided on the core support plate of the reactor then becomes difficult. In practice, the assembly to be inserted is laterally abutted against an assembly already in position and it is lowered, at a slow speed to reduce friction, while visually monitoring that there is no catching between adjacent grids. If it is found that the slope which the assembly takes due to the offset is too great, it is turned about a vertical axis so as to abut the opposite face. Such operations lengthen refueling and are a source of trouble.